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RE: VR Tubes for Screens

Posted by Janos (A) on April 21, 2021 at 16:27:50

In Reply to: VR Tubes for Screens posted by Triode_Kingdom on March 22, 2021 at 23:16:58:

I've been using a pair of 0D3 VR tubes in series for 807 screens for decades (807 is similar o 6L6 - screen can handle lower voltages, but plate can handle higher).
In SE, a single pair of 0D3 tubes (supplying 300V reg) is sufficient for a one 807 per channel. (But not sufficient for a pair of them per channel.)

Technically a 3nF capacitance is the maximum a VR tube can handle without starting to oscillate, but practically you will be fine up to 6-8nF or so. (6nF for one VR, but put them in series for higher voltage and the capacitance goes down to half!)

However, you can slap on as much capacitance as you want if you de-Q the load. (VR tubes have negative resistance, which causes them to oscillate with higher capacitances.)

Here's how de-Q goes for VR tubes:
1. Add your desired capacitor for bypass, connect leads as close to tube screen and cathode resistor ground connection as possible.(Yes, you can use 10uF or more if you want - however, the bigger you use, the slower the supply gets, try to go for a high quality 0,22uF film cap)I recommend as high quality bypass cap as possible, it will have a big impact on the sound.
2. between the bypass cap and the VR tubes add a 10-100R resistor and a series capacitor. Add this bypass pair to between VR output + and -. The exact resistor value does not matter, but the capacitance has to be higher than the bypass cap. (Eg if you use 0,22uF for the bypass on the screen, then this cap has to be 0,33uF or 0,47uF - or 5uF, whatever.)
The addition of (2.) will prevent the VR tube from going into oscillation. I've used th higher capacitance scenario for a phono stage PSU with 10uF / 30R-50uF, and that gave a super quiet B+ (On a pair of 0D3 giving 300V), suitable for phono stage quiet. :)

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